Destiny
When Arthur leads his allies to invade Rome, he believes that he is acting in accordance with what is right, proper, and in fact, his appropriate destiny. He has two prophetic dreams. The first is a dream about a battle between a dragon and a bear, which his advisors tell him predicts a battle between himself and a giant, with Arthur emerging victorious. Soon afterward, Arthur has an opportunity to fight a giant who raped and murdered one of Guenevere's female relatives. Arthur defeats the giant, credibly supporting the notion that his dreams predict the future.
Arthur's second dream features a woman representing Fortune or luck. She -- the only speaking female character in the entire poem -- explains to Arthur that he has a good run of luck but that it will soon be time for the good times to end and for him to die. This scene foreshadows Arthur's injury in battle and the coup by Mordred, who claims Arthur's throne and wife in his absence.
What it means to be a "good" king
In the earlier Beowulf, a good king was a strong fighter who distributed his wealth freely among retainers, who did not hoard wealth or influence himself, but who shared it. To a certain extent the Alliterative Morte Arthure follows this tradition. Arthur, who wins in single combat with the giant and who physically leads his army into battle instead of coordinating strategy from the rear the way most modern generals do, is more similar to a Geatish King from Beowulf than he is to most medieval rulers who delegated the actual management of their armies to others. The later Sir Gawain and the Green Knight features Arthur as a more passive ruler who allows one of his knights, specifically Sir Gawain, to accept a challenge in his stead.
Compared to the Emperor of Rome, who has to hire mercenaries and pay for people to fight alongside him (suggesting that his cause is not worth supporting, or that he himself was not worth supporting, Arthur enjoys the support of allies who come freely and of their own will. This suggests that Arthur not only has remarkable alignment of interests with his allies, but that he has a remarkable cult of personality. All good, virtuous, and reasonable people support him. In fact, he is the late medieval equivalent of a "Mary Sue" or "Gary Stu" character as described in modern fan fiction: a perfect character with no real flaws who does not really make mistakes.
The just war
The narrator spends hundreds of lines explaining why the invasion of the Roman Empire was a just, appropriate, and indeed necessary thing for Arthur and his allies to do. The envoys from the Emperor were rude, arrogant, belligerent, unreasonable, and lacking in courtesy. By contrast, Arthur presented himself as cool-headed, reasonable, and superbly courteous: he offers the envoys safe passage out of his lands even after he has decided to go to war. He even goes so far as to explain what his route and strategy will be, so that his enemy can prepare himself. His knights, in discussing reasons to go to war, cite ancient crimes committed by the Romans and other reasons to wage vengeful war.
The fact that Arthur's invasion is successful is regarded by both the narrator and Arthur's supporters as evidence that it is supported not just by reason and the best judgement of human beings, but by divine will.
